Hitherto, there have been proposed various transistors or diodes having a semiconductor region in which the width of a forbidden band (namely, a band gap) is graded in a tapered like form and are effective in speeding up the frequency response and photoresponse, in the case of using them as a phototransistor or a photodiode.
However, for these transistors or diodes, studies have forcused on using a crystal semiconductor, particularly GaAs (Al) semiconductor wherein can be, a transistor or a diode is prepared in accordance with molecular beam epitaxy method. [see, F. Capasso, Surface Science, 142, pp. 513-528 (1984)]
In the molecular beam epitaxy method, the film forming operation is practiced in an ultra-high vacuum atmosphere, and the deposition rate of the semiconductor film formed on a substrate is slow. In addition, it is not only difficult to mass-produce such film but also to produce large square films. Further, Ga and As raw materials are troublesome since they are harmful to man.
Other than the above, it has been attempted to prepare such semiconductor devices using easily obtainable Si and Ge as raw materials. However, it is commonly recognized that it is difficult to make a single crystal film which is free from undesired structural defects using such raw materials since the grading constants of Si and Ge are different from each other.
In this respect, studies have focused on a non-single-crystal SiGe film which is usable for the preparation of a solar cell and a photosensor. In the case of this non-single-crystal film, there are advantages such that it is not necessary to consider the above problems relating to such differences among the constituent materials, the structural freedom is large, dangling bonds can be easily compensated with hydrogen atom or halogen atom such as fluorine, and because of all of this, an objective non-single-crystal SiGe film can be effectively formed.
In addition, the band gap of the non-single-crystal SiGe film, can be continuously varied by changing the proportions of Si and Ge contained therein.
Likewise, various studies have been made also on non-single crystal SiC, SiN and SiO films, which are usable for the preparation of the above mentioned semiconductor devices.
For such non-single-crystal films, it is also possible to continuously grade their band gaps by changing the proportions of their constituent elements.
However, it has not yet become possible to obtain a desired highly efficient transistor, diode, etc. by using these non-single-crystal films because of their low mobilities although there is a proposal for the preparation of a transistor or photodiode having a hetero junction using such non-single-crystal film as disclosed in U.S. Pat. No. 4,254,429 which is aimed at preventing defects or/and mismatches from the interface between constituent layers. Because of this, even in that publication, a highly efficient transistor or diode has not yet been attained.
Referring to the semiconductor devices disclosed in U.S. Pat. No. 4,254,429, when a semiconductor film in which both the conduction band and the valence band are inclined against a Fermi level and which has a band gap expanded towards the other direction, that is to say, while forming a funnel-like shaped band gap, the device characteristics may be raised since a carrier of either a hole or an electron is apt to be accumulated.
For instance, in the case where said device is employed as a transistor, it has low transistor characteristics or where it is employed as a diode, such diode has low diode characteristics.
Further, in the case where said device is employed as a solar cell, it is not possible to desirably increase any of short circuit current (Isc), open circuit voltage (Voc) and Fill factor (FF).